The so-called permanently excited synchronous machine is an example of such an electric machine. Permanently excited synchronous machines are used in numerous applications in which electric drive tasks are to be carried out. In industrial applications, for example for machine tools or production machines, they are used as highly dynamic servomotors. Due to their high power density compared to other types of machines, they are also preferably used in the area of electromobility, in which the available installation space often represents a limiting variable. However, the permanently excited synchronous machine is also frequently used as a generator, for example in the field of regenerative energies, in particular wind power.
In comparison to electrically excited synchronous machines, the permanently excited synchronous machine is characterized by an increased efficiency. Ohmic losses are saved due to the fact that the permanently excited synchronous machine may dispense with electrical excitation. The excitation field of the machine is generally generated by permanent magnets situated in the rotor of the machine. A slip ring contact, which is necessary in electrically excited synchronous machines in order to supply current to an excitation coil situated on the rotor, may be dispensed with in the permanently excited synchronous machine. As a result, the maintenance effort for the permanently excited machine is also reduced compared to the electrically excited machine.
However, one disadvantage of permanent-magnet excitation is that the excitation field is not easily modifiable. In principle, a synchronous machine may be operated beyond its nominal speed by controlling the so-called field weakening range. In this range, the machine is operated at the maximum nominal power, the torque delivered by the machine being reduced with increasing rotational speed. Electrically excited synchronous machines may be operated very easily in the field weakening range by reducing the exciting current.
Options are known, also for permanently excited machines, for generating, via suitable energization of the stator of the machine, an air gap field component which counteracts the excitation field generated by the permanent magnets and thus weakens the excitation field. However, such control of the machine results in increased losses, so that the machine may be operated only at a reduced efficiency in this range.
To be able to operate permanently excited dynamoelectric machines in the field weakening range without appreciably impairing the efficiency of the machine, methods for mechanical field weakening are known from the prior art. Thus, CN 101783536 A describes a permanently excited synchronous motor with buried permanent magnets that are magnetized in the tangential direction, and which are adjoined in each case by a short circuit block that is radially displaceable, viewed radially outwardly. This short circuit block is pretensioned via a spring in such a way that the former is situated in a magnetically insulating area of the rotor at a low rotor rotational speed. With increasing rotational speed, the short circuit block is pressed outwardly against the spring tension, where it forms a short circuit path for the magnetic flux. The magnetic leakage flux guided over this short circuit path reduces the effective air gap flux of the machine, so that the field weakening operation is controlled.